Theoretical Calculations of Infrared Bands of CH₃⁺and CH₅⁺

Abstract

Existing theoretical calculations predict that infrared spectra of the two most fundamental reactive carbo-ions, methyl cation CH3 + with D3h symmetry and protonated methyl cation CH5 + with Cs(I), Cs(II), and C2v symmetries, appear together in the 7-μm region corresponding to the C-H bending modes. Vibrational band profiles of CH3 + and CH5 + have been compared by ab initio calculation methods that use the basis sets of MP2/ aug-cc-pVTZ and CCSD(T)/cc-pVTZ. Our results indicate that the bands of rotation-vibration transitions of CH3 + and CH5 + should overlap not only in the 3-μm region corresponding to the C-H stretching modes but also in the 7-μm region corresponding to the C-H bending modes. Five band intensities of CH5 + among fifteen vibrational transitions between 6 and 8 μm region are stronger than those of the ν2 and ν4 bands in CH3 + . Ultimate near degeneracy of the two bending vibrations ν2 and ν4 of CH3+ along with the stronger intensities of CH5 + in the three hydrogen scrambling structures may cause extreme complications in the analysis of the highresolution carbo-ion spectra in the 7-μm region.